Method for producing aluminum structure and aluminum structure

ABSTRACT

An object is to provide a method for producing an aluminum structure using a porous resin body having a three-dimensional network structure, with which an aluminum structure having a low impurity content can be formed, and in particular, a porous aluminum body having a large area can be obtained. 
     A method for producing an aluminum structure includes a conductivity-imparting step of applying a conductive coating material containing conductive carbon onto a surface of a resin body to impart electrical conductivity to the resin body; a plating step of plating a surface of the resin body, to which electrical conductivity has been imparted, with aluminum in a molten salt to form an aluminum layer; and a heat treatment step of conducting heat treatment to remove the resin body, wherein the conductive carbon is carbon black having an average particle diameter of 0.003 μm or more and 0.05 μm or less.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of International Application No. PCT/JP2012/063007, filed May 22, 2012, which claims priority to Japanese Patent Application No. 2011-124707, filed Jun. 3, 2011. The contents of these applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an aluminum structure suitable for use as a porous metal body used in various filters, battery electrodes, etc. and a method for producing the aluminum structure.

BACKGROUND ART

Porous metal bodies having a three-dimensional network structure are used in various applications such as filters, catalyst supports, and battery electrodes. For example, Celmet (manufactured by Sumitomo Electric Industries, Ltd.: registered trademark) composed of nickel is used as an electrode material of a battery such as a nickel-metal hydride battery or a nickel-cadmium battery. Celmet is a porous metal body having continuous pores, and has a feature that the porosity thereof is higher (90% or more) than that of other porous bodies such as metal nonwoven fabrics. Celmet is produced by forming a nickel layer on a surface of a skeleton of a resin foam body having continuous pores, such as urethane foam, decomposing the resin foam body by heat treatment, and then conducting a reduction treatment on nickel. The nickel layer is formed by performing a conductivity-imparting treatment by applying a carbon powder or the like onto the surface of the skeleton of the resin foam body, and then depositing nickel by electroplating.

Aluminum is excellent in terms of electrical conductivity and corrosion resistance, and is a lightweight material. Regarding the applications of aluminum to batteries, for example, an aluminum foil whose surface is coated with an active material such as lithium cobalt oxide is used as a positive electrode of a lithium-ion battery. One conceivable method for improving the capacity of a positive electrode is to process aluminum into a porous body so as to increase the surface area and to fill the interior of the porous aluminum body with an active material. This is because, with this structure, the active material can be efficiently utilized even in an electrode having a large thickness, and the ratio of utilizing the active material per unit area can be improved.

As for a method for producing a porous aluminum body, PTL 1 describes a method for forming a metallic aluminum layer of 2 to 20 μm on a plastic base having inner continuous spaces and a three-dimensional network shape by performing an aluminum vapor deposition process by an arc ion plating method. PTL 2 describes a method for obtaining a porous metal body by forming a coating film of a metal (such as copper), which will form a eutectic alloy at a temperature equal to or lower than the melting point of aluminum, on a skeleton of a resin foam body having a three-dimensional network structure, then applying an aluminum paste thereto, and conducting heat treatment at a temperature of 550° C. or higher and 750° C. or lower in a non-oxidizing atmosphere to eliminate an organic component (resin foam) and sinter an aluminum powder.

As for plating of aluminum, it is difficult to conduct aluminum electroplating in an aqueous solution-based plating bath because aluminum has high affinity to oxygen and an electric potential lower than that of hydrogen. Therefore, for aluminum electroplating, non-aqueous solution-based plating baths have been studied. For example, as for a technique of aluminum plating for the purpose of preventing oxidation of a metal surface or the like, PTL 3 discloses a method of electroplating of aluminum in which a low-melting-point composition prepared by mixing and melting an onium halide and an aluminum halide is used as a plating bath and aluminum is deposited on a cathode while maintaining the water content in the bath at 2% by weight or less.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 3413662

[PTL 2] Japanese Unexamined Patent Application Publication No. 8-170126

[PTL 3] Japanese Patent No. 3202072

SUMMARY OF INVENTION Technical Problem

PTL 1 describes that a porous aluminum body having a thickness of 2 to 20 μm is obtained by the method disclosed therein. However, it is difficult to produce a porous aluminum body having a large area because a gas phase method is employed, and it is also difficult to form a layer that is uniform even in the interior of the base depending on the thickness and the porosity of the base. In addition, this method has problems in that the rate of formation of the aluminum layer is low and the production cost increases because the equipment is expensive. Furthermore, in the case of the formation of a thick film, cracks may be formed in the film and the aluminum film may be detached. According to the method disclosed in PTL 2, a layer that forms a eutectic alloy with aluminum is formed and thus an aluminum layer having a high purity cannot be formed. Although aluminum electroplating methods are known, only metal surfaces can be plated by the methods and a method for electroplating a surface of a resin body, in particular, a method for electroplating a surface of a porous resin body having a three-dimensional network structure, has not been known. It is believed that this is due to problems such as dissolution of porous resins in plating baths.

The inventors of the present invention have conceived, as a method with which aluminum plating can be conducted even on a porous resin body having a three-dimensional network structure and a porous aluminum body having a high purity can be formed by uniformly forming a thick film, a method for producing a porous aluminum body including imparting electrical conductivity to a surface of a resin body having a three-dimensional network structure and composed of polyurethane, a melamine resin, or the like, and then plating aluminum in a molten-salt bath, and have filed an application for a patent. Examples of the method for imparting electrical conductivity to a surface of a resin body include non-electrolytic plating of a conductive metal such as nickel, deposition of a metal such as aluminum by a gas phase method (such as sputtering or vapor deposition), and application of a conductive coating material containing conductive particles such as carbon particles. After the plating of aluminum, the resin body is removed. Thus, an aluminum structure mainly composed of aluminum is obtained.

When electrical conductivity is imparted to a resin body using a metal other than aluminum, such as nickel, the metal other than aluminum remains as an impurity in the resulting aluminum structure. For use where the purity of aluminum is required, e.g., for use as a battery electrode, since the presence of such an impurity prevents satisfactory characteristics from being realized, this method for imparting electrical conductivity is not suitable. An aluminum structure having a high purity can be produced by imparting electrical conductivity using aluminum. However, in order to impart electrical conductivity using aluminum, it is necessary to employ a gas phase method such as vapor deposition or sputtering, and thus the production cost is increased.

The application of a conductive coating material containing conductive carbon is a relatively easy method, and production can be performed at a low cost. In addition, a metal other than aluminum, such as nickel, does not remain. However, in the case where electrical conductivity is imparted by using conductive carbon, it is difficult to completely remove the conductive carbon in a step of removing a resin body after a plating step of aluminum, and carbon remains as an impurity in the resulting aluminum structure. When the amount of carbon remaining in the aluminum structure is increased, the aluminum structure is easily broken from a starting point due to the residual carbon, which may cause a decrease in the strength of the aluminum structure. Residual carbon may also cause welding defects in a step of preparing a battery electrode.

Accordingly, an object of the present invention is to provide a method for producing an aluminum structure using a resin body, in particular, a porous resin body having a three-dimensional network structure, the method being capable of preparing an aluminum structure having a low impurity content, and being capable of obtaining an aluminum structure having a large area and, in particular, suitable for use as an electrode.

Solution to Problem

The present invention provides a method for producing an aluminum structure, the method including a conductivity-imparting step of applying a conductive coating material containing conductive carbon onto a surface of a resin body to impart electrical conductivity to the resin body; a plating step of plating a surface of the resin body, to which electrical conductivity has been imparted, with aluminum in a molten salt to form an aluminum layer; and a heat treatment step of conducting heat treatment to remove the resin body, wherein the conductive carbon is carbon black having an average particle diameter of 0.003 μm or more and 0.05 μm or less.

Hitherto, in order to impart electrical conductivity to a resin body in producing nickel Celmet or the like, graphite having an average particle diameter of about 1.5 μm has been used as conductive carbon. In producing of nickel Celmet, a resin body is removed in a high-temperature atmosphere at about 600° C. to 800° C. in the air, and a reduction treatment is further conducted at 1,000° C. In such a high-temperature atmosphere, conductive carbon can be satisfactorily decomposed and removed even when graphite having a relatively large average particle diameter is used. However, the melting point of aluminum is 660° C., and it is necessary to remove a resin body at a temperature equal to or lower than this temperature. Furthermore, aluminum is easily oxidized, and once aluminum is oxidized, a reduction treatment cannot be performed at a temperature equal to or lower than the melting point. Thus, the heat treatment temperature is preferably low. As a result of studies on the type of conductive carbon that can be satisfactorily removed by such a low-temperature treatment, it was found that carbon can be satisfactorily removed by a treatment at a relatively low temperature and an aluminum structure having a low residual carbon content can be obtained by using conductive carbon black that does not have crystallinity but is amorphous and that has an average particle diameter of 0.003 μm or more and 0.05 μm or less.

The heat treatment step is preferably conducted at a temperature of 500° C. or higher and 640° C. or lower in an atmosphere containing oxygen. When the temperature exceeds 640° C., oxidation of aluminum easily proceeds. Thus, when the resulting aluminum structure is used as an electrode material for batteries, current collecting characteristics decrease. When the temperature is lower than 500° C., the amount of residual conductive carbon increases. The heat treatment temperature is more preferably 580° C. or higher and 620° C. or lower. When the heat treatment step is conducted in an atmosphere containing oxygen, conductive carbon can be removed within a short time.

In particular, a resin body having a complex skeleton structure, such as a porous resin body having a three-dimensional network structure, may be used. In this case, an aluminum structure having a high porosity can be obtained and suitably used in application to an electrode, etc. In addition, the resin body is preferably composed of polyurethane, which can provide a porous resin body having a high porosity and can be satisfactorily decomposed in the heat treatment step.

An aluminum structure is produced through the above steps. The aluminum structure has a high purity, and a carbon content of the aluminum structure may be 2% by weight or less. The carbon content in the aluminum structure can be measured by a high-frequency combustion infrared absorption method using a high-frequency induction heating furnace.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a method for forming an aluminum structure having a low impurity content using a resin body, in particular, a porous resin body having a three-dimensional network structure, and the aluminum structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing steps of producing an aluminum structure according to the present invention.

FIG. 2 includes schematic cross-sectional views illustrating steps of producing an aluminum structure according to the present invention.

FIG. 3 is an enlarged photograph of a surface showing the structure of urethane foam, which is an example of a porous resin body.

FIG. 4 is a view illustrating an example of a continuous step of imparting electrical conductivity to a surface of a resin body by using a conductive coating material.

FIG. 5 is a view illustrating an example of a continuous step of aluminum plating by molten-salt plating.

FIG. 6 is a schematic cross-sectional view illustrating a structural example in which a porous aluminum body is applied to a molten-salt battery.

FIG. 7 is a schematic cross-sectional view illustrating a structural example in which a porous aluminum body is applied to an electrical double layer capacitor.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described. In the drawings referred to below, parts that are assigned the same numeral are the same or corresponding parts. Note that the present invention is not limited to the embodiments but defined by the claims, and is intended to include all modifications within the scope and meaning of equivalents of the claims.

Steps of Producing Aluminum Structure

FIG. 1 is a flowchart showing steps of producing an aluminum structure according to the present invention. FIG. 2 schematically illustrates steps of forming the aluminum structure using a resin body as a core material in accordance with the flowchart. The overall flow of the production steps will be described with reference to the two figures. First, preparation 101 of a resin body serving as a base is conducted. FIG. 2( a) is an enlarged schematic view of an enlarged surface of a resin foam body having continuous pores. Pores are formed by a resin foam body 1 serving as a skeleton. Next, impartation of electrical conductivity 102 to the surface of the resin body is conducted. In this step, a thin, electrically conductive layer 2 is formed on the surface of the resin body 1, as illustrated in FIG. 2( b). Subsequently, aluminum plating 103 in a molten salt is conducted to form an aluminum plating layer 3 on the surface of the resin body having the electrically conductive layer thereon (FIG. 2( c)). Thus, an aluminum-coated resin body in which the aluminum plating layer 3 is formed on the surface of the resin body serving as the base is prepared. Subsequently, removal 104 of the resin body is conducted. The aluminum-coated resin body is heat-treated to decompose and remove the resin foam body 1, thereby obtaining an aluminum structure (porous body) constituted by only the metal layer (FIG. 2( d)). These steps will be sequentially described below.

Preparation of Porous Resin Body

A resin foam body composed of polyurethane and having a three-dimensional network structure and continuous pores is prepared. A resin body having any shape may be selected as long as the resin body has pores that are continuous (continuous pores). For example, a nonwoven fabric including entangled resin fibers may also be used instead of the resin foam body. The resin foam body preferably has a porosity of 80% to 98% and a pore diameter of 50 to 500 μm. Urethane foam is preferably used as the resin foam body because urethane foam has a high porosity and pore continuity, and is good in terms of uniformity of pores.

A resin foam body often contains residues such as a foaming agent and an unreacted monomer in the process of producing the foam. Therefore, a washing treatment is preferably performed for the subsequent steps. FIG. 3 shows, as an example of a resin foam body, urethane foam that has been subjected to a washing treatment. The resin body serving as a skeleton constitutes a network three-dimensionally, thereby forming continuous pores as a whole. The skeleton of polyurethane foam has a substantially triangular shape in a cross section perpendicular to a direction in which the skeleton extends. Herein, the porosity is defined by the following formula.

Porosity=(1−(weight of porous material [g]/(volume of porous material [cm³]×density of raw material))×100[%]

The pore diameter is determined by magnifying a surface of the resin body by means of a photomicrograph or the like, counting a cell number per inch (25.4 mm), and calculating an average value as mean pore diameter=25.4 mm/cell number.

Impartation of Electrical Conductivity to Resin Body Surface: Application of Conductive Coating Material

A conductive coating material is prepared in which carbon black having an average particle diameter of 0.003 μm or more and 0.05 μm or less is used as conductive carbon. The conductive coating material is a suspension containing conductive carbon, a binder, a dispersant, and a dispersion medium. In order to uniformly apply conductive particles, it is necessary that the suspension maintain a uniform suspension state. Accordingly, the suspension is preferably maintained at 20° C. to 40° C. This is because when the temperature of the suspension is lower than 20° C., a uniform suspension state is impaired, and only the binder is concentrated on a surface of the skeleton constituting a strip-shaped structure of the resin body and forms a layer. In such a case, the applied layer of carbon particles is easily separated and it is difficult to form metal plating that tightly adheres to the layer of carbon particles. On the other hand, when the temperature of the suspension exceeds 40° C., the amount of evaporation of the dispersion medium is large. Accordingly, the suspension is concentrated with the lapse of the application process time, and the amount of carbon applied tends to vary.

Carbon black which is amorphous carbon is used as the conductive carbon. The average particle diameter of the conductive carbon is 0.003 μm or more and 0.05 μm or less, and more preferably 0.005 μm or more and 0.02 μm or less. When the average particle diameter is excessively large, decomposability in a heat treatment step decreases. When the average particle diameter is excessively small, it is difficult to ensure sufficient electrical conductivity. Note that the average particle diameter is a value calculated from a specific surface area measured using a specific surface area measuring device.

The porous resin body can be coated with carbon particles by immersing a target resin body in the suspension and squeezing and drying the resin body. FIG. 4 is a view that schematically illustrates, as an example of a practical production process, the structure of a treatment apparatus that imparts electrical conductivity to a strip-shaped porous resin body serving as a skeleton. As illustrated in the figure, this apparatus includes a supply bobbin 12 that supplies a strip-shaped resin 11, a vessel 15 that contains a suspension 14 of a conductive coating material, a pair of squeeze rolls 17 arranged above the vessel 15, a plurality of hot air nozzles 16 provided so as to face each other on the sides of the travelling strip-shaped resin 11, and a take-up bobbin 18 that takes up the strip-shaped resin 11 after a treatment. Deflector rolls 13 for guiding the strip-shaped resin 11 are arranged at appropriate positions. In the apparatus having the above structure, the strip-shaped resin 11 having a three-dimensional network structure is unwound from the supply bobbin 12, guided by the deflector rolls 13, and immersed in the suspension in the vessel 15. The strip-shaped resin 11 immersed in the suspension 14 in the vessel 15 is turned upward and travels between the squeeze rolls 17 above the liquid surface of the suspension 14. At this time, the gap between the squeeze rolls 17 is smaller than the thickness of the strip-shaped resin 11 and the strip-shaped resin 11 is compressed. Thus, excess suspension impregnated in the strip-shaped resin 11 is squeezed out and returns to the vessel 15.

Subsequently, the direction in which the strip-shaped resin 11 travels is changed again. Here, the dispersion medium etc. in the suspension are removed by hot air blown from the hot air nozzles 16, which are constituted by a plurality of nozzles, and the strip-shaped resin 11 that are thoroughly dried is taken up by the take-up bobbin 18. The temperature of the hot air blown from the hot air nozzles 16 is preferably in the range of 40° C. to 80° C. With the apparatus described above, a conductivity-imparting treatment can be conducted automatically and continuously and a skeleton having a network structure free of clogging and a uniform conductive layer is formed. Thus, metal plating, which is the subsequent step, can be smoothly conducted.

Formation of Aluminum Layer: Molten-Salt Plating

Next, electrolytic plating is conducted in a molten salt to form an aluminum plating layer 3 on the surface of the resin body. A direct current is supplied between the resin body having a surface to which electrical conductivity is imparted, the resin body serving as a cathode, and an aluminum plate having a purity of 99.99% and serving as an anode in a molten salt. The aluminum plating layer has a thickness of 1 to 100 μm, and preferably 5 to 20 μm. The molten salt may be an organic molten salt that is a eutectic salt of an organohalide and an aluminum halide or an inorganic molten salt that is a eutectic salt of an alkali metal halide and an aluminum halide. An organic molten-salt bath that melts at a relatively low temperature is preferably used because plating can be performed without decomposition of a resin body serving as a base. An imidazolium salt, a pyridinium salt, or the like can be used as the organohalide. Specifically, 1-ethyl-3-methylimidazolium chloride (EMIC) and butylpyridinium chloride (BPC) are preferred. A salt containing an imidazolium cation having alkyl groups at the 1- and 3-positions is preferably used as the imidazolium salt. In particular, a molten salt of aluminum chloride and 1-ethyl-3-methylimidazolium chloride (AlCl₃-EMIC) is most preferably used because it has high stability and is not easily decomposed.

Mixing of moisture or oxygen into the molten salt degrades the molten salt. Therefore, plating is preferably conducted in an inert gas atmosphere such as nitrogen or argon in a closed environment. In the case where an EMIC bath is used as an organic molten-salt bath, the temperature of the plating bath is 10° C. to 60° C., and preferably 25° C. to 45° C.

In the case where an imidazolium salt bath is used as a molten-salt bath, an organic solvent is preferably added to the molten-salt bath. Xylene is particularly preferably used as the organic solvent. Addition of an organic solvent in particular, xylene, achieves advantages specific to the formation of a porous aluminum body. Specifically, it is possible to obtain a first feature that the aluminum skeleton forming a porous body is not easily broken and a second feature that uniform plating can be performed in which a difference in plating thickness between a surface portion and an inner portion of the porous body is small. The first feature is due to the fact that the addition of an organic solvent improves the shape of plating on the skeleton surface from a granular state (which is significantly uneven and appears to be granules in surface observation) to a flat shape, thereby increasing the strength of the skeleton having a small thickness and a small width. The second feature is due to the fact that the addition of an organic solvent to a molten-salt bath decreases the viscosity of the molten-salt bath and the plating bath easily passes through the inner portion of the fine network structure. More specifically, when the viscosity is high, a fresh plating bath is easily supplied to the surface of the porous body, but is not easily supplied to the inner portion. In contrast, by decreasing the viscosity, the plating bath is easily supplied to the inner portion, and thus plating that provides a uniform thickness can be performed. The amount of organic solvent added to the plating bath is preferably 25% to 57% by mole. When the amount of organic solvent is 25% by mole or less, it is difficult to achieve the effect of reducing the difference in plating thickness between a surface layer and an inner portion. When the amount of organic solvent is 57% by mole or more, the plating bath becomes unstable and a plating solution and xylene are partially separated.

Furthermore, subsequent to the step of conducting plating using the molten-salt bath containing an organic solvent, a washing step in which the organic solvent is used as a washing liquid is preferably performed. It is necessary to wash a surface of a plated resin in order to wash away a plating bath. Such washing after plating is usually performed with water. However, it is essential that moisture be avoided in an imidazolium salt bath. If washing is performed with water, water is taken in a plating solution in the form of water vapor or the like. Thus, washing with water should be avoided in order to prevent adverse effects on plating. Accordingly, washing with an organic solvent is effective. Furthermore, in the case where an organic solvent is added to a plating bath as described above, a more advantageous effect can be obtained by conducting washing with the organic solvent added to the plating bath. Specifically, the washed plating solution can be relatively easily recovered and reused, and the cost can be reduced. For example, it is supposed that a plated body formed in a bath prepared by adding xylene to a molten salt AlCl₃-EMIC is washed with xylene. The resulting liquid after washing is a liquid that contains xylene in an amount larger than that of the plating bath that is originally used. A certain amount or more of the molten salt AlCl₃-EMIC is not mixed with xylene. Thus, the liquid after washing is separated into xylene on the upper side and the molten salt AlCl₃-EMIC containing about 57% by mole of xylene on the lower side. Therefore, the molten liquid can be recovered by collecting the separated liquid on the lower side. Furthermore, since the boiling point of xylene is as low as 144° C., the xylene concentration in the recovered molten salt is adjusted to the xylene concentration in the plating solution by applying heat, and the resulting solution can be reused. After the washing with an organic solvent, it is also preferable to further conduct washing with water in another place that is separated from the plating bath.

FIG. 5 is a view that schematically illustrates the structure of an apparatus for continuously conducting a metal plating treatment on a strip-shaped resin. This figure illustrates a structure in which a strip-shaped resin 22 having a surface to which electric conductivity has been imparted is transported from the left to the right of the figure. A first plating vessel 21 a includes a cylindrical electrode 24, a positive electrode 25 provided on the inner wall of the container, and a plating bath 23. The strip-shaped resin 22 passes through the plating bath 23 along the cylindrical electrode 24. Thus, an electrical current can evenly and easily flows in the entire resin body and uniform plating can be obtained. A second plating vessel 21 b is a vessel for further forming thick and uniform plating and is configured so that plating is repeatedly performed in a plurality of vessels. Plating is conducted by allowing the strip-shaped resin 22 having a thin metal layer on a surface thereof to pass through a plating bath 28 while sequentially feeding the strip-shaped resin 22 using electrode rollers 26 functioning as both transfer rollers and out-of-vessel power-supply negative electrodes. Positive electrodes 27 are provided in the plurality of vessels so as to face two surfaces of the resin with the plating bath 28 therebetween. With this structure, the two surfaces of the resin can be coated with a more uniform plating film.

Decomposition of Resin: Heat Treatment

Through the above steps, an aluminum-coated resin body including a resin body as a core of the skeleton is prepared. Next, the resin body is removed. The aluminum-coated resin body is heat-treated at a temperature of 500° C. or higher and 640° C. or lower to decompose the resin body and conductive carbon. When the heat treatment is conducted in the presence of oxygen, a urethane decomposition reaction easily proceeds and the conductive carbon can also be satisfactorily decomposed. Heat treatment performed with a gas flow is preferable because decomposition products are effectively removed.

Lithium-Ion Battery

Next, a battery electrode material and a battery that use an aluminum structure will be described. For example, when an aluminum structure is used in a positive electrode of a lithium-ion battery, lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), lithium nickel oxide (LiNiO₂), or the like is used as an active material. The active material is used in combination with a conductive aid and a binder. A known positive electrode material for a lithium-ion battery is obtained by coating a surface of an aluminum foil with an active material. The thickness of the active material coating is increased in order to improve the battery capacity per unit area. In order to effectively utilize the active material, it is necessary that the aluminum foil and the active material electrically contact with each other, and thus the active material is mixed with a conductive aid. In contrast, the aluminum structure of the present invention has a high porosity and a large surface area per unit area. Accordingly, the active material can be effectively utilized even when an active material having a small thickness is supported on the surface of the aluminum structure, the capacity of the battery can be improved, and the amount of conductive aid mixed can be reduced. In a lithium-ion battery, the positive electrode material described above is used as the positive electrode, graphite is used as the negative electrode, and an organic electrolyte solution is used as an electrolyte. Such a lithium-ion battery can have an improved capacity even when it has a small electrode area. Therefore, the energy density of the battery can be made higher than the energy densities of known lithium ion batteries.

Molten-Salt Battery

An aluminum structure can also be used as an electrode material for a molten-salt battery. When a porous aluminum body is used as a positive electrode material, a metal compound that can intercalate a cation of a molten salt serving as an electrolyte, for example, sodium chromate (NaCrO₂) or titanium disulfide (TiS₂)), is used as the active material. The active material is used in combination with a conductive aid and a binder. Acetylene black and the like can be used as the conductive aid. Polytetrafluoroethylene (PTFE) and the like can be used as the binder. When sodium chromate is used as the active material and acetylene black is used as the conductive aid, PTFE is preferred because it can more firmly bond the two substances to each other.

An aluminum structure can also be used as a negative electrode material of a motel-salt battery. When a porous aluminum body is used as the negative electrode material, elemental sodium, an alloy of sodium and another metal, carbon, or the like can be used as the active material. Since the melting point of sodium is about 98° C. and the metal softens with an increase in temperature, sodium is preferably alloyed with another metal (such as Si, Sn, or In). Among these, an alloy of sodium and Sn is particularly preferable because the alloy is easy to handle. Sodium or a sodium alloy can be supported on the surface of the porous aluminum body by electrolytic plating, hot dipping, or the like. Alternatively, a sodium alloy can be formed by depositing a metal (such as Si) to be alloyed with sodium on a porous aluminum body by plating or the like and then conducting charging in the molten-salt battery.

FIG. 6 is a schematic cross-sectional view illustrating an example of a molten-salt battery that uses the above-described electrode material for a battery. The molten-salt battery includes a positive electrode 121 in which a positive electrode active material is supported on the surface of the aluminum skeleton portion of an aluminum structure, a negative electrode 122 in which a negative electrode active material is supported on the surface of the aluminum skeleton portion of an aluminum structure, and a separator 123 impregnated with a molten salt serving as an electrolyte. The positive electrode 121, the negative electrode 122, and the separator 123 are housed in a case 127. A pressing member 126 including a pressing plate 124 and a spring 125 that presses the pressing plate is arranged between the upper surface of the case 127 and the negative electrode. Since the pressing member is provided, even when the positive electrode 121, the negative electrode 122, and the separator 123 are subjected to volume changes, all the components can be uniformly pressed and brought into contact with each other. A collector (porous aluminum body) of the positive electrode 121 and a collector (porous aluminum body) of the negative electrode 122 are respectively connected to a positive electrode terminal 128 and a negative electrode terminal 129 through lead wires 130.

Various inorganic salts or organic salts that melt at an operating temperature can be used as the molten salt serving as an electrolyte. At least one selected from alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) and alkaline earth metals such as beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba) can be used as the cation of the molten salt.

In order to decrease the melting point of the molten salt, two or more salts are preferably used as a mixture. For example, when potassium bis(fluorosulfonyl)amide (KFSA) and sodium bis(fluorosulfonyl)amide (NaFSA) are used in combination, the operating temperature of the battery can be controlled to be 90° C. or lower.

The molten salt is used by impregnating the separator. The separator is provided in order to prevent the positive electrode and the negative electrode from contacting each other. A glass nonwoven fabric, a porous resin, and the like can be used as the separator. The positive electrode, the negative electrode, and the separator impregnated with the molten salt are stacked, housed in the case, and used as a battery.

Electrical Double Layer Capacitor

An aluminum structure can also be used as an electrode material for an electrical double layer capacitor. When an aluminum structure is used as an electrode material for an electrical double layer capacitor, activated carbon or the like is used as an electrode active material. Activated carbon is used in combination with a conductive aid and a binder. Graphite, carbon nanotubes, and the like can be used as the conductive aid. Polytetrafluoroethylene (PTFE), styrene-butadiene rubber, and the like can be used as the binder.

FIG. 7 is a schematic cross-sectional view illustrating an example of an electrical double layer capacitor that uses the above-described electrode material for an electrical double layer capacitor. An electrode material in which an electrode active material is supported on an aluminum structure is arranged as polarizable electrodes 141 in an organic electrolyte solution 143 partitioned by a separator 142. Each of the polarizable electrodes 141 is connected to a lead wire 144. All of these components are housed in a case 145. By using a porous aluminum body as a collector, the surface area of the collector is increased. Thus, an electrical double layer capacitor that can realize high output and high capacity can be produced even when activated carbon serving as an active material is thinly applied.

A description has been made of a case where a resin foam body is used as a resin body, but the present invention is not limited to a resin foam body. An aluminum structure having any shape can be obtained by using a resin body having any shape.

Formation of Electrically Conductive Layer: Example 1

A production example of an aluminum structure will be specifically described below. A urethane foam having a thickness of 1 mm, a porosity of 95%, and the number of pores per centimeter of about 20 was prepared as a resin foam body and cut into a 15 mm×15 mm square. The urethane foam was immersed in a carbon suspension and dried to form an electrically conductive layer, the entire surface of which had carbon particles adhering thereon. The suspension contained, as components, 80% by weight of conductive carbon black having an average particle diameter of 0.01 μm, a resin binder serving as a binder, a penetrant, an antifoamer, and a dispersion medium.

Formation of Electrically Conductive Layer: Comparative Example 1

A urethane foam having a thickness of 1 mm, a porosity of 95%, and the number of pores per centimeter of about 20 was prepared as a resin foam body and cut into a 15 mm×15 mm square. The urethane foam was immersed in a carbon suspension and dried to form an electrically conductive layer, the entire surface of which had carbon particles adhering thereon. The suspension contained, as components, 80% by weight of graphite having an average particle diameter of 1.5 μm, a resin binder serving as a binder, a penetrant, an antifoamer, and a dispersion medium.

Formation of Electrically Conductive Layer: Comparative Example 2

A urethane foam having a thickness of 1 mm, a porosity of 95%, and the number of pores per centimeter of about 20 was prepared as a resin foam body and cut into a 15 mm×15 mm square. The urethane foam was immersed in a carbon suspension and dried to form an electrically conductive layer, the entire surface of which had carbon particles adhering thereon. The suspension contained, as components, 80% by weight of graphite having an average particle diameter of 1.0 μm, a resin binder serving as a binder, a penetrant, an antifoamer, and a dispersion medium.

Molten-Salt Plating

Each of the urethane foams produced in Example 1, Comparative Example 1, and Comparative Example 2 and having an electrically conductive layer on the surface thereof was set to a fixture having a power-supplying function, and then immersed in a molten salt aluminum plating bath (67 mol % AlCl₃-33 mol % EMIC) at a temperature of 40° C. The fixture to which the urethane foam was set was connected to the cathode side of a rectifier, and an aluminum plate (purity: 99.99%) serving as a counter electrode was connected to the anode side. Plating was conducted for 90 minutes at a current density of 3.6 A/dm². Herein, the current density is a value calculated on the basis of the apparent area of the urethane foam. As a result, an aluminum plating layer having a weight of 150 g/m² could be formed.

Decomposition of Resin Foam Body

The resin foam bodies each having an aluminum plating layer thereon were heat-treated at a temperature of 600° C. for 30 minutes in the air atmosphere to prepare aluminum structures of Example 1, Comparative Example 1, and Comparative Example 2. The residual carbon content of each of the aluminum structures was measured by a high-frequency combustion infrared absorption method. The residual carbon content of the aluminum structure of Example 1 was low; 1.3% by weight (2.0 g/m²). In contrast, the residual carbon content of Comparative Example 1 was 5.5% by weight (8.2 g/m²) and the residual carbon content of Comparative Example 2 was 3.0% by weight (4.5 g/m²).

The above description encompasses other embodiments described below.

Other Embodiment 1

An electrode material in which an active material is supported on an aluminum surface of an aluminum structure obtained by the present invention.

Other Embodiment 2

A battery in which the electrode material described in Other Embodiment 1 is used in at least one of a positive electrode and a negative electrode.

Other Embodiment 3

An electrical double layer capacitor in which the electrode material described in Other Embodiment 1 is used as an electrode.

Other Embodiment 4

A filtration filter including an aluminum structure obtained by the present invention.

Other Embodiment 5

A catalyst support in which a catalyst is supported on a surface of an aluminum structure obtained by the present invention.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a porous aluminum structure can be obtained. Thus, for example, the present invention can be widely used in various fields in which characteristics of aluminum are utilized, for example, in electric materials such as battery electrodes, filters for various types of filtration, and catalyst supports.

REFERENCE SIGNS LIST

-   1 resin foam body 2 electrically conductive layer 3 aluminum plating     layer 11 strip-shaped resin 12 supply bobbin 13 deflector roll 14     suspension 15 vessel 16 hot air nozzle 17 squeeze roll 18 take-up     bobbin -   21 a, 21 b plating vessel 22 strip-shaped resin -   23, 28 plating bath 24 cylindrical electrode -   25, 27 positive electrode 26 electrode roller -   121 positive electrode 122 negative electrode 123 separator 124     pressing plate 125 spring 126 pressing member 127 case 128 positive     electrode terminal 129 negative electrode terminal 130 lead wire -   141 polarizable electrode 142 separator 143 organic electrolyte     solution 144 lead wire 145 case 

1. A method for producing an aluminum structure comprising: a conductivity-imparting step of applying a conductive coating material containing conductive carbon onto a surface of a resin body to impart electrical conductivity to the resin body; a plating step of plating a surface of the resin body, to which electrical conductivity has been imparted, with aluminum in a molten salt to form an aluminum layer; and a heat treatment step of conducting heat treatment to remove the resin body, wherein the conductive carbon is carbon black having an average particle diameter of 0.003 μm or more and 0.05 μm or less.
 2. The method for producing an aluminum structure according to claim 1, wherein the heat treatment step is conducted at a temperature of 500° C. or higher and 640° C. or lower in an atmosphere containing oxygen.
 3. The method for producing an aluminum structure according to claim 1, wherein the resin body is a porous resin body having a three-dimensional network structure.
 4. The method for producing an aluminum structure according to claim 1, wherein the resin body is composed of polyurethane.
 5. An aluminum structure produced by the method according to claim
 1. 6. An Aluminum structure according to claim 5, wherein a carbon content is 2% by weight or less. 